Intersubband relaxation in modulation-doped multiple quantum well structures

A thoretical investigation of the time constants of intersubband transitions in n-type GaAs/Al_x Ga_1-x As modulation-doped multi-quantum-well (MD MQW) structures by emission of polar optical phonons is presented. It is shown that the space charge resulting from positively ionized donors induces conduction band minima in the barriers that contain several bound states. In narrow wells, the upper subband interacts with the eigenstates of the barrier and causes a delocalization of the upper subband wave function into the AlGaAs barrier. This interaction leads to increasing probability for excited carriers to reside in the barriers and relax to the ground state with emission of AlGaAs LO phonons. Thus transition rates to the ground state are calculated for both wells and barriers. Although experimental and theoretical lifetimes are slightly different, the factor of 2 decrease in relaxation time seen experimentally is also predicted theoretically.